1. Field of the Invention
Embodiments of the present invention generally relate to a process for using a dimethyl carbonate and methanol mixture. In particular, embodiments of the present invention relate a process of using a dimethyl carbonate and methanol azeotropic mixture. In particular still, embodiments of the present invention relate to a process of using a dimethyl carbonate mixture in a reaction to produce a carbonate.
2. Description of the Related Art
Dimethyl carbonate (also referred to herein as “DMC”) is useful as a starting material in the synthesis of carbonates and polycarbonates. Dimethyl carbonate is often present in a mixture with methanol. In various processes to produce carbonates, dimethyl carbonate is often obtained in a mixture with methanol. The mixture of dimethyl carbonate and methanol form an azeotropic composition when the composition includes approximately 70 wt % methanol and 30 wt % DMC at atmospheric pressure.
Various processes have been proposed to separate the azeotropic mixture and reuse the DMC. Known processes for separating the azeotropic mixture of dimethyl carbonate and methanol include extractive distillation, membrane separation, and pressure distillation.
In an extractive distillation process, an extractive agent is added to the mixture of components to be separated in the distillation process. The addition of the extractive agent changes the relative volatilities of the components of the mixture. The relative volatilities are changed sufficiently to allow effective separation by distillation. For example, U.S. Pat. No. 5,292,917 discloses a process for the extractive distillation of a dimethyl carbonate/methanol mixture by distilling the mixture in the presence of dimethyl oxalate. Two distillation columns are used in series to separate the dimethyl carbonate from the methanol. In the first column, methanol is removed at the top of the column, and the dimethyl carbonate/dimethyl oxalate mixture is fed to the second column. In the second column, the dimethyl carbonate is separated from the dimethyl oxalate.
Membranes have also been proposed for the separation of methanol and DMC. However, the purity of the streams obtained do not always justify the expense of the separation and in many cases still require some sort of distillation.
Pressure distillation can be used to lower the DMC concentration in the azeotrope. However, a pressure of 150 psig is typically needed to sufficiently break the azeotrope. Columns rated at 150 psig operating pressure are not as common as low pressure or vacuum towers. It is also not typical to find a pressure tower attached to a batch reactor.
There is a need, therefore, for a simpler and more economical process for utilizing a dimethyl carbonate and methanol mixture. There is also a need for a process for using the dimethyl carbonate and methanol mixture as a reactant to produce a desired product, such as a carbonate or carbamate.